February 01, 2011

Multi-regionalism or assimilation?

In Is Multi-regional evolution dead? I argue that recent work on Neandertal/Denisovan admixture in modern humans has been misinterpreted as signifying only a limited contribution of archaic hominins to the dominant "Out of Africa" population. The main points of my argument are:

Each non-modern specimen so far has yielded evidence of admixture; and since we have only examined only 2 types of archaic hominins in a relatively small part of Eurasia, it stands to reason that additional hominin populations (in Africa and the rest of Eurasia) may have also contributed their DNA to the modern human gene pool

The uniformity of the common population element in modern humans should not be interpreted as exclusively African in origin. In a multi-regional model with gene flow we expect parts of the genome to be derived from different regions. The residual "admixture" with archaics may represent the constituent elements that have not gone global.

Ann Gibbons, writing in Science repeats the claim in favor of assimilationism:

New DNA data from archaic human species are providing a much higher resolution view of our past. When compared with the genomes of living people, the ancient genomes allow anthropologists to thoroughly test the competing models of human origins for the first time. The DNA data suggest not one but at least two instances of interbreeding between archaic and modern humans, raising the question of whether Homo sapiens at that point was a distinct species (see sidebar). And so they appear to refute the idea that modern humans came out of Africa, spread around the world, and completely replaced the archaic humans they met. But the genomic data also don't prove the classic multiregionalism model, which argues that a single, worldwide species of human, including archaic forms outside of Africa, met, mingled, and had offspring, and so produced Homo sapiens. They suggest only a small amount of interbreeding, presumably at the margins where invading moderns met archaic groups. The new picture most resembles so-called assimilation models, which got relatively little attention over the years.

This is a fallacious argument: even if we assume that the sub-10% admixture represents the entirety of non-African archaic admixture, this does not mean that there was a "small amount" of interbreeding "at the margins".

It may very well reflect differences in population sizes of different Pleistocene hominin populations.

Consider two species A and B, and a composite species C having 95% of genes derived from A and 5% of genes derived from B. This may either mean that there was sporadic and exceptional intermixture between A- with B-poeple. Alternatively, it may mean that there were many more A than B people, and even though nearly all B people were absorbed, they still did not affect the major group substantially.

This is not an insignificant point: occasional admixture at the fringes is quite a different phenomenon than wholesale admixture between demographically unequal populations.

I don't have any data on population sizes in the last 100-200 thousand years, but I would venture that before the technological advances of the last 50 thousand years ago, when full behavioral modernity arrived, northern Eurasians (Neandertals or Denisovans) did not have as great population densities and sizes as could be maintained in either the African homeland of Homo or more temperate climate zones in general. So, I would lean towards the idea that the observed levels of admixture signify substantial admixture between demographically unequal populations, rather than exceptional admixture.

20 comments:

I have to repeat here my argument that all: climate, fossils, and genetics favor that until about 300,000 years ago there indeed was sufficient admixture and gene flow between Europe, West Asia, and Africa to cause significant co-development of important characteristics (such as brain case size and behavior).

East Asia happened to not be part of this, and climate conspired to cause a break in this until the last expansions out of a wet North Africa (130,000 ya and perhaps a couple of times, after).

This doesn't make modern humans multi-regional. They are primarily European/West Asian/African between 800,000 and 300,000 years ago, and after that African with a couple of European / New Asian admixtures.

For a long time I have accepted the genetic-based estimates that least half the human population lived in Africa until close to the end of the Pleistocene, and that the total census size of the human species did not exceed 1-2 million years until <50,000 years. This is totally in agreement with estimates from the archaeological record. I would only add to this excellent BLOG that Smith notes the assimilation theory is a limited case of Mutiregional evolution.

If we think of modern humans, Neanderthals and Denisovans as members of the same species, admixture seems to us more normal and even expected than in the classical Out of Africa scenario. All we need to do is to change our perspectives.

"that before the technological advances of the last 50 thousand years ago, when full behavioral modernity arrived, northern Eurasians (Neandertals or Denisovans) did not have as great population densities and sizes as could be maintained in either the African homeland of Homo or more temperate climate zones in general. So, I would lean towards the idea that the observed levels of admixture signify substantial admixture between demographically unequal populations, rather than exceptional admixture."

I agree that Neanderthals and Denisovans most likely were organized into small, widely-dispersed demes subject to genetic drift and showing deep substructure. Population size east of the Movius line was much lower than population size west of the Movius Line throughout Lower Pleistocene. I also think that modern Amerindians preserved, until recently, this Lower-Middle Pleistocene demographic structure better than any other modern human population.

But if we suppose that there's a useful analogy between the colonization of the Americas from 1492 on - during which a large, growing immigrant population pushed, absorbed and mixed with small indigenous populations - and the colonization of the Old World by incoming Homo sapiens in the Late Pleistocene, we should expect to find Homo sapiens genetic markers in Neanderthals and Denisovans, exactly like we find tons of European admixture in modern Amerindians. So far none of that Homo sapiens-to- Neanderthals/Denisovans gene flow has been reported.

Regarding population structure in North Africa, the Nm values tended to infinity for most pairs of North African samples, indicating the lack of any barrier to migration in the region. The only exception are the High Atlas Moroccans (Asni), which show a low rate of gene flow as compared to the remaining North African samples in agreement with the extreme position of this population in the PC plots (Figure 3)

http://www.biomedcentral.com/1471-2148/10/84

EastAsian looking,but, without any EastAsian genetic ingredient, Strange no ?

"In a multi-regional model with gene flow we expect parts of the genome to be derived from different regions. The residual 'admixture' with archaics may represent the constituent elements that have not gone global".

It's good to read that. I don't feel near as lonely as I did five or six years ago when I seemed to be the only person with that view.

"If we think of modern humans, Neanderthals and Denisovans as members of the same species, admixture seems to us more normal and even expected than in the classical Out of Africa scenario. All we need to do is to change our perspectives".

Great to see the idea becoming more accepted. I very much suspect that we can regard everything from H. erectus to today as being a single species. With regional, or subspecies, variation of course.

"the assimilation theory is a limited case of Mutiregional evolution".

I find it difficult to see a distinction between the two.

"if say a homosapien x entered neanderthal early 50k after separation it would be able to fit, if then separate for 50k, the homosapien x carried by the neanderthal may be the only one to biologically fit back in".

After just one generation the two X chromosomes of the two species would be mixed. They would not still be two discrete chromosomes.

"So far none of that Homo sapiens-to- Neanderthals/Denisovans gene flow has been reported".

We would have trouble distinguishing such admixture from commonality of origin though. Genes not found in chimpanzees but present in both archaic and modern humans could have moved in either direction.

I hope Salawusu man could be the next sample of ancient DNA, he probably got few mixture with Neanderthals like Denisovans if we judge that according to his anthropological traits. Link:http://archaeology.about.com/od/sterms/g/salawusu.htm

Not sure what your point is, but obviously North Africa has had a very turbulent past even in the last ~2,000 years, alone. Besides a Saharan and sub-Saharan component, I see in your images the history of west Asian (Eastern and Southeastern Mediterranean) and European settlements. It would be interesting to see if there is anything here that signals/ pre-dates the development of Caucasian features outside of the regions between Europe and Pakistan/India - but I think we really need to go back and look at actual finds from 30,000 - 130,000 ya to answer that one, instead of looking at present-day pictures.

(1) the total absence of non-African dervived mtDNA and Y-DNA in any modern humans. One can imagine all uniparental traces disappearing much more easily in an occasional admixture scenario (particularly if they were different species cause Haldane's law to apply), than in a total absorbsion scenario. We have examples of modern human human total absobsion cases where there was not elimination of mtDNA and Y-DNA traces of prior populations, and

(2) the pre-Neolithic population density of modern humans during the time period where there was overlap of Neanderthals and modern humans (ca. 30,000 years ago), was probably not 20-1 greater than the Neanderthal population. Hunting and gathering without herd animals, horses, dogs, or reliable deep water boats, is a very low population density activity, even if you are very good at it, and the overlap areas would have been inland and hence not benefitted from the higher population densities that fishing can sometimes afford. Modern humans may have had an edge, but probably nowhere close to that much of an edge, and

(3) Neanderthal culture was static for a very long time and was quite different from that of the modern humans they encountered. It is hard to see the entire population of Neanderthals being flexible enough to adapt to a modern human lifestyle, and the skeletal evidence that there were large and sustained mixed Neanderthal and modern human communities with any frequency isn't there, particularly in the places (SE Asia, SW Asia) where admixture would have to have taken place to match the genetic mixes that we see today. There is a gap in skeletons and there is a gap in tool kits in the archaeological record. There are a handful of ambiguous sites and it may have happened in isolated instances with some individuals, but there is nothing in the archaeological record to go beyond the mere possibility from genetics that it could have happened that way. Most of the 30,000 year old +/- sites are still pretty clearly one or the other.

The Out of the Americas (OoAm) proposal reverses the commonly held assumption of a “Peopling of the Americas” and converts it into the testable hypothesis of a New World speciation event (or genuine autochthonous origins) that led to the formation of Homo sapiens in the Americas with subsequent colonization of the Old World by the new hominid species? This single-origin alternative suggests that the “sudden replacement” of Old World Homo erectus populations is compatible with a New World source for Homo sapiens, with all human ancestors being the product of an evolutionary process exclusive to the Western Hemisphere. Reasonable scientific objectives should, in the very least, compel researchers to follow German Dziebel's example (often expressed on Dienekes and Razib's Discover blogs) to seriously evaluate this alternative. An alternative set of “points of order” starts with identifying historical and scientific discretion encompassing the evolution of Old World human origin theories, themselves, directed by their own “points of order” which have inadvertently nullified the evaluation of this premise (side effects of the Clovis First model that requires scientists to integrate the significance of a pre-projectile point horizon with a later arrival of “Paleoindian Industries”). The evolution itself to Upper Paleolithic-like projectile industries accompanies the 43,000 ybp expansions Old World H sapiens, into Europe from Siberia (Kara-Bom, etc.). Out of the Americas (OoAm) presents fresh insights into the seemingly unending controversy overshadowing the prospects of finding a resolution to our human past’s evolutionary wellspring. OoAm suggests that isolation from the Old World is compatible with separate homelands for Homo sapiens and Homo erectus. It is the only option proponents of “Sudden Replacement” have never addressed while it would offer a compatible alternative, nigh neutralizing, “Multi-regional evolution” by isolating our species “in only one area of the World”. The OoAm hypothesis has not been adequately addressed but it does satisfy the requirements for a single-origin alternative as advanced by Wolpoff:

“In contrast, multiregional evolution can easily be disproved if it can be shown that all of the ancestors of living humans at some discrete time in the Middle or Late Pleistocene lived in only one area of the world. If this were the case, then we should be able to trace the ancestry of every human genetic locus to a single population existing at some time in the past million years.” (Milford Wolpoff et al. pg.131 Multiregional, Not Multiple Origins, in AJPA 112:129-136 (2000) (emphasis added)

Out of America is a nonsensical theory that makes no sense and has no empirical evidence in its support. Native Americans have neither the older fossils (they have the youngest), nor the oldest autosomal diversity (again one of the youngest), nor the oldest Y-chromosomes/mtDNA (again they belong to a few derived clades relative to the rest of mankind.

Dienekes, you are funny. There's no such thing as "youngest fossils". They are not trees. There are only fossils that have been found to date and those that have not been found yet. The Zhirendong fossil instantly made the East Asian human fossil record older by a whopping 60,000 years. What's important is that during the 100 years of purposeful digging we haven't found any skulls or tools in Asia that can be thought of as direct progenitors of the earliest Amerindian skulls and tools. It's now certain that the occupation of the Americas is older than the Clovis horizon (see Monte Verde, Paisley Cave, etc.). We just don't know by how much.

Regarding genes, I posted on Razib's a list of genetic findings that are suggestive of Amerindian antiquity and compatible with an out of America scenario, via a Eurasian hominid ancestor. http://blogs.discovermagazine.com/gnxp/2011/02/friday-fluff-%E2%80%93-february-4th-2011/#more-9697

Please take off your eyes of the genes and bones for a second and look at languages and kinship studies. Out of America is a hypothesis just like any other, including your African substructure idea or the idea that Africans are more diverse genetically because they are more admixed with African archaic hominids or that humans contributed genes to Neanderthals. How is that for mainstream views?

Dienekes, you are funny. There's no such thing as "youngest fossils". They are not trees. There are only fossils that have been found to date and those that have not been found yet. The Zhirendong fossil instantly made the East Asian human fossil record older by a whopping 60,000 years.

The gap between the earliest attested American H. sapiens and the earliest attested African H. sapiens is about 190ky, so you have your work cut out for you.

Not to mention that in the Americas there is not only an absence of skulls, but also an absence of archaeological evidence for the presence of man before ~15ky.

Please take off your eyes of the genes and bones for a second and look at languages and kinship studies

Languages are useless beyond the Neolithic. It's ridiculous to use a very malleable, fast-evolving, and noise system (language) when there is a much better one (DNA).

The Out-of-America theory is pure nonsense, and I have tolerated long enough its evidence-free promotion in this blog.

"The gap between the earliest attested American H. sapiens and the earliest attested African H. sapiens is about 190ky, so you have your work cut out for you."

We have work cut out for all of us to explain the gap between 190K, when supposedly our biological ancestors emerged, and 40K, when worldwide evidence for modern human behavior - your behavior included - emerged. Don't assume that it's just self-explanatory.

"Not to mention that in the Americas there is not only an absence of skulls, but also an absence of archaeological evidence for the presence of man before ~15ky."

So we have a gap between 15K in America and 40K elsewhere for modern human tools, which is 25K only. Up until 1980s, people thought Australia was peopled only 10K years ago.

"Languages are useless beyond the Neolithic. It's ridiculous to use a very malleable, fast-evolving, and noise system (language) when there is a much better one (DNA)."

You are just incompetent about languages and overcompetent about DNA. Try to put the scales in balance. Did languages emerged from the breath of God 5,000 years ago? There are 140 language stocks in America and only 20 in Africa. Just test the hypothesis of Amerindian antiquity using language data - it would be more productive than to simply dismiss it. You did work on skulls and genes. Take on languages.

"The Out-of-America theory is pure nonsense, and I have tolerated long enough its evidence-free promotion in this blog."

I'm a reader and user of this blog just like anyone else. We all have our ideas and our data.

A Theory embracing "extensive genetic diversity" and "mutational-drift equilibrium" in Amerindian tribal populations (Ward et al. 1991; Chakraborty and Weiss 1991; and others) contrast favorably with the limited genetic diversity supporting the Eve hypothesis for Old World populations. The so called Out of the Americas Theory does not necessitate nor direct that mtDNAs ABCD be part of the original exodus, only M and N as the basal component. Alternatively, the suggested presence in Northeast Asia of founding or nodal mitochondrial DNA lineages for each “proposed” Amerindian haplogroup (Torroni et al. 1993a) precludes an alternative explanation, that of more recent Holocene admixture between Northeast Asians (OoAm) and later Amerindians. This second migration out of the Americas provides a backdrop for the later formation of Sea Mammal Hunting Cultures, an idea Franz Boas identified as "Eskimo wedge theory" (Boas 1905 and 1910; Ousley and others in, Human Biology, June 1995). Presumably, as Boas believed, the removal of glacial barriers allowed human contact between the Longitudinal Hemispheres that was geographically un-encumbered and evidenced by widespread Holocene acculturation (also see Ackerman 1982; Dumond 1983; and Heizer 1943; and others) inasmuch as earlier Amerindian tribal populations existed before deglaciation. An alternative to a “Dramatic Founding Effect… ” is a theory embracing a pre-Clovis presence as part of the equation. A mid-Pleistocene presence south of the Ice sheets (not in Beringia) compliments a later Amerindian/Paleoindian contribution to Eskimo, Athapaskan, and as well, Siberians Population formation/assimilation since these groups share "distinct genetic affinities with Native Americans (Torroni et al. 1993b, pg. 591).” Dienekes “again they belong to a few derived clades relative to the rest of mankind”mtDNAs A, B, C, and D, do not branch off ancestral Old World M, N, or R branches/twigs but are recent arrivals to the Old World and thus, basal in appearance. Simply, they do NOT branch off derived sets of any of the Old World M, N, or Rs, something one would expect with the Americas being the most recently “peopled” continents. As for mtDNA X2, this Lineage fits within an Old World branching order (see Phylotree) indicating that it could have arrived in Late Pleistocene/Early Holocene times while its primarily northeastern American distributions suggests admixture with pre-existing Amerindian populations. In keeping with this post … Possible assimilation, not decent from the Neandertal, may explain the 1%-4-8% factor but a separate origin for Homo sapiens best explains the missing 92 to 99%, among other possibilities.

"We have work cut out for all of us to explain the gap between 190K, when supposedly our biological ancestors emerged, and 40K, when worldwide evidence for modern human behavior - your behavior included - emerged".

To me the evidence is overwhelming that the OoA is not associated with the possession of 'modern human behavior'. That behaviour, as you say, emerged around 40kya. whereas the OoA may be as old as 100kya. The mistake most students of human evolution have been making is in trying to tie the two together. It can't be done.

"To me the evidence is overwhelming that the OoA is not associated with the possession of 'modern human behavior'. That behaviour, as you say, emerged around 40kya. whereas the OoA may be as old as 100kya."

To me the evidence is insufficient to demonstrate that an out of Africa migration happened at all, hence it's a moot point whether this "ghost-migration" was associated with a behavioral revolution. A behavioral revolution, on the other hand, is supported by a myriad of uncontroversial facts such as the unprecedented level of technological sophistication, impact on the environment, the disappearance of previous hominid species, the proliferation of languages all over the globe, etc. All of these indications of behavioral revolution(s) are worldwide. Archaeologically, the first indications of a new type of behavior that has resulted in all these changes is only 40K years old.

The mistake that most people make is divorcing the archaeological study of the roots of "culture" from the study of the present (or pre-1492) cultural and linguistic diversity. If we look at culture holistically across archaeology and ethnology, we'll see two poles of cultural "expression": the Euro-African one characterized by a wealth of Upper Paleolithic and Late Stone Age artifacts from 40K on but low levels of linguistic and cultural diversity (only 20 language stocks in Africa) and the Asian-Amerindian-Australasian one characterized by the paucity of the former (in America only 15K) and the extreme diversification of the latter (140 language stocks in America).

Both dimensions are complementary in our study of "behavioral modernity." Unfortunately, they remain largely divorced from each other, hence people misinterpret New World archaeological record as evidence of the absence of modern humans, while linguistic diversities as the product of Neolithic miracles.

Dienekes said “Not to mention that in the Americas there is not only an absence of skulls, but also an absence of archaeological evidence for the presence of man before ~15ky.”Does the American pre-Clovis offer behavioral comparisons with either Middle Paleolithic and/or Upper Paleolithic activities? What seems to be missing are less modified stone tools, leaving hearths and other imported stone and bone tools and as a common singularity demarking all Amerindian mid-Pleistocene/pre-Clovis archaeological contexts. From Confronting the Neanderthals “Instead we agree with Bar-Yosef who reiterates a point he has made before (1987,34) that 'it is not in the lithics, but in features such as hearths, the spatial distribution of debris, etc., that the emergence of modern humans is reflected'. (pg. 114 Christopher Stringer & Clive Gamble)”Hearth-like features found in other older pre-Clovis sites are often dismissed without reason as the radiocarbon dates are too old to accept or ponder the Worldly significance. Again, a combination of paradigm conflict and “instant analysis” are at play when dismissing archaeological features found in pre-Clovis sites. Another comparison from the Old World can be appropriated. If only the New World hearths were naturally created then one would have to expect that similar features would be found in Middle Paleolithic Europe or elsewhere in the Old World. Yet, hearths or other supposed’ naturally created hearth-like features are not found in pre-modern hominid locals. If they had been discovered scientific journals would be awash with them with the “instant analysis” being… ‘look-here, sapient activity before the Upper Paleolithic’. Simply, it is rock or clay lined hearths that distinguish a human element to pre-Clovis sites while it is now expected that we will not find rock or clay lined hearth features in association with Pleistocene Homo erectus activities. The most promising archaeological signatures supporting a human presence at MV-I (at 33k y.b.p.) are the two hearth features and imported/selected stone tools with virtually identical compositions to the later 15k y.b.p MV II.

“The features are located between 60 and 75 meters due south of the present-day creek and buried in a rise or promontory of an ancient marsh or stream (see Chapter 2; Figs. 6.3-6.4 pg.89).” from Monte Verde: A Late Pleistocene Settlement in Chile, by Tom Dillehay Washington, DC: Smithsonian Institution Press, 1997.

“The feature fill consists of 67.4 percent clay, 23.9 Percent sand, 1.4 percent wood charcoal, 1.0 percent seeds of juncus sp. and 2.1 percent small clasts… both burned and unburned seeds and wood were recovered (Fig. 6.6)…. Karathanasis’s study detected high portions of quartz and feldspar in the clay, which is expected in this depositional environment (see appendix I). He also found significant amounts of phosphate mineral apatite, zink, and minor amounts of Fe-hydroxide (geothite). He attributes the phosphates to bone residues. The zinc could be the result of decayed plant matter but more likely of animal remains. He found some structural similarity between the fill of Feature 1 and those of clay lined features DW-23 and D-8 in Areas DW and D, Zone D, in the MV-II younger component. The similarities suggest that the clays from the two different levels were derived from similar and possibly the same clay source.” (Dillehay 1997:91)

“Lastly, as mentioned above, there is no hard evidence to indicate these [hearths] features are the products of natural agencies. There is no geological evidence to indicate nonconformity or other mechanisms that might have eroded a previous surface and left the clay features as lag deposits. There is no evidence to indicate animal burrowing or burned tree stumps and other phenomena. Excavations in the deeper levels of trenches and blocks did not yield any features or anomalies similar to the features described above. (Dillehay 1997:94)”

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